fixes #40, extension methods for primitive types

test/codegen/expect/cascade.js

 var cascade = dart.defineLibrary(cascade, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
   function test_closure_with_mutate() {
@@ skipping 31 matching lines @@
     let a = null, b = null;
     a = new A();
     a.x = b = null;
     a.x = b = null;
     a = null;
     core.print(a);
   }
   dart.fn(test_mutate_outside_cascade, dart.void, []);
   function test_VariableDeclaration_single() {
     let a = [];
-    a[core.$length] = 2;
-    a[core.$add](42);
+    a.length = 2;
+    a[dartx.add](42);
     core.print(a);
   }
   dart.fn(test_VariableDeclaration_single, dart.void, []);
   function test_VariableDeclaration_last() {
     let a = 42, b = (() => {
       let _ = [];
-      _[core.$length] = 2;
-      _[core.$add](a);
+      _.length = 2;
+      _[dartx.add](a);
       return _;
     })();
     core.print(b);
   }
   dart.fn(test_VariableDeclaration_last, dart.void, []);
   function test_VariableDeclaration_first() {
     let a = (() => {
       let _ = [];
-      _[core.$length] = 2;
-      _[core.$add](3);
+      _.length = 2;
+      _[dartx.add](3);
       return _;
     })(), b = 2;
     core.print(a);
   }
   dart.fn(test_VariableDeclaration_first, dart.void, []);
   function test_increment() {
     let a = new A();
     let y = ((() => {
       a.x = dart.dsend(a.x, '+', 1);
       a.x = dart.dsend(a.x, '-', 1);
       return a;
     })());
   }
   dart.fn(test_increment, dart.void, []);
   let Base$ = dart.generic(function(T) {
     class Base extends core.Object {
       Base() {
-        this.x = dart.setType([], core.List$(T));
+        this.x = dart.list([], T);
       }
     }
     return Base;
   });
   let Base = Base$();
   class Foo extends Base$(core.int) {
     Foo() {
       super.Base();
     }
     test_final_field_generic(t) {
-      this.x[core.$add](1);
-      this.x[core.$add](2);
-      this.x[core.$add](3);
-      this.x[core.$add](4);
+      this.x[dartx.add](1);
+      this.x[dartx.add](2);
+      this.x[dartx.add](3);
+      this.x[dartx.add](4);
     }
   }
   dart.setSignature(Foo, {
     methods: () => ({test_final_field_generic: [dart.void, [core.Object]]})
   });
   // Exports:
   exports.A = A;
   exports.test_closure_with_mutate = test_closure_with_mutate;
   exports.test_closure_without_mutate = test_closure_without_mutate;
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 })(cascade, core);